In fill level measurement, microwaves are transmitted by means of an antenna onto the surface of a fill substance, and the echo waves reflected at the surface are received. An echo function is formed, representing the echo amplitudes as a function of distance. The echo function is then used to determine the probable useful echo and its travel time. From the travel time, the separation between the fill substance surface and the antenna is determined.
In the technology of industrial measurements, dielectric rod antennas and horn antennas are regularly used for the transmitting and/or receiving. Frequently, a housing is applied, which exhibits a housing section having the geometry of a short-circuited waveguide.
An exciter element extends into the housing section with such waveguide geometry, and microwaves are transmitted and/or received through the section over the exciter element. In the case of transmitting, the microwaves are produced by a remotely arranged microwave generator and transported to a transmitting element, or to a transmitting- and receiving-element, over coaxial conductors. In the antenna, there occurs by way of the transmitting element, or the transmitting- and receiving-element, a conversion of fed, conductor-bound microwaves into microwaves that propagate in free space, and vice versa.
In the case of a horn antenna, there follows on the housing a funnel-shaped section widening in the container-facing direction to form the horn. In the case of the rod antenna, a rod of a dielectric material is provided, pointing into the container. Usually, the interior of the housing is almost completely filled by an insert of a dielectric. In the case of the horn antenna, the insert has a cone-shaped end pointing into the container. In the case of rod antennas, the rod-shaped antenna pointing into the container is connected to the insert.
The housing is regularly made of metal and will hereinafter be referred to as the outer conductor. The dielectric insert and its cone-shaped end, or the dielectric insert and the rod-shaped antenna connected thereto, as the case may be, will hereinafter be referred to as the inner conductor.
FIG. 1 shows schematically a container 1, the left side of which shows a fill level measuring device 3 equipped with a horn antenna and the right side of which shows a fill level measuring device 5 equipped with a rod antenna.
If chemically aggressive, easily ignitable or explosive media are present in the container, then a pressure-tight and gas-sealed closure of the interior of the container is required. The same is true, when the container is pressurized. Correspondingly, the inner conductor must be anchored pressure-tightly and gas-sealed in the outer conductor, in order that the container interior be completely separated from the container exterior.
Since the inner conductor is made of a dielectric and the outer conductor of a metal, different materials having different coefficients of thermal expansion come together here.
U.S. Pat. No. 5,877,663 describes a fill level measuring device working with microwaves, having an antenna for transmitting, or for transmitting and receiving, microwaves, which has a metallic outer conductor and a ceramic inner conductor. The inner conductor is secured in the outer conductor by means of a press- or shrink-fit, and, since these methods alone offer an inadequate level of safety, additionally by means of brazing.